Target concentration and separation using magnetic beads (hereinafter MBs), for their unique advantages, have been gaining popularity in biochemical practice including microfluidics. These techniques may be used for purification of bodily fluids. See e.g., O. Olsvik, T. Popovic, E. Skjerve, K. S. Cudjoe, E. Homes, J. Ugelstad, and M. Uhlen, “Magnetic Separation Techniques in Diagnostic Microbiology”, Clinical Microbiology Reviews, Vol. 7, pp. 43-54, 1994. Still others have employed magnetic to isolate specific cells from blood samples. See e.g., V. I. Furdui and D. J. Harrison, “Immunomagnetic T Cell Capture From Blood For PCR Analysis Using Microfluidic Systems,” Lab on a Chip, Vol. 4, pp. 614-618, 2004. In a typical procedure, the MBs are held back (i.e., trapped) on a surface position by magnetic force, while a fluid passes by or over the MBs. When the fluid contains targets, these targets build on the MB surface and thus become concentrated, as the MBs have a special affinity to the targets. When the fluid is exchanged with a wash fluid, all the particles and species except the targets on the MBs are washed away and separated (i.e., the targets on the MBs are purified). In the case of a typical prior art immunoassay, the target is a protein, and the MBs are conjugated with antibody (Ab). This prior art technique is described in FIGS. 1A-G. As seen in FIG. 1A, MBs conjugated with Ab are introduced as a suspension in a liquid. As seen in FIG. 1B, the antibody (Ab)-conjugated MBs are then trapped at a location on the surface of the device using an external magnet.
Next, as seen in FIG. 1C, a sample of proteins, that includes both the target protein and non-target proteins, are permitted to flow into the device and past the antibody (Ab)-conjugated MBs trapped at the surface of the device. Referring now to FIG. 1D, target proteins bind to the Ab that is conjugated to the MBs. Unbound non-target proteins are washed away with a wash solution.
Optionally, as seen in FIG. 1E, labeled antibody (Ab) is then permitted to flow into the device. The labeled Ab will then bind to those Ab-conjugated MBs that are already bound with target protein. In this regard, the labeled Ab will sandwich the target protein. The labeled Ab may include a fluorescent label that fluoresces in response to incident radiation.
Unbound labeled Ab is then washed away as illustrated in FIG. 1F. Detection or analysis or any other processes may optionally be performed on the sample. The magnet may then be removed as illustrated in FIG. 1G, and the MBs and targets (as well as, optionally, their conjugated labels) may then be washed away from the surface of the device, for further processes or operations with this purified sample.
The above prior art process applies to conventional conditions, where liquids move between containers in bulk or flow continuously within channels. However, when the fluids are handled in digitized packets (e.g., droplets), as in droplet-based digital microfluidics, the above-noted process does not work because of the existence of liquid-gas or liquid-liquid interfaces. In particular, as the droplet passes across the position where the MBs are to be held, the passing meniscus pulls the MBs away from the surface with a force that is orders-of-magnitude stronger than the magnetic force. The magnetic force produced by even with the strongest magnet available is not strong enough to counter the interfacial force in the scale of typical MBs. Because of this strong interfacial force (i.e., surface tension) of the meniscus of the droplet, the MBs cannot be magnetically trapped on the surface against a droplet sliding across the surface. Instead, the MBs get carried away with the moving droplet. The interfacial force is also much stronger than the short range surface forces (e.g., van der Waals force) which act on MBs in contact with the device surface. This is true for commercially available “naked” MBs (e.g. polystyrene beads) as well as antibody-conjugated MBs often used for cell selection and protein enrichment, among many other cases. This, the prior art process envisioned in FIGS. 1A-1G is inapplicable for droplet-based microfluidic applications.